For Mastodon: this isn’t the same as you are saying with the user-follow. In your case, each users follows the users that they want to see, and they can partecipate in the comments of that post, but they will see ALL of the posts of that user in their timeline/feed. Here, the community choose what users/hashtags/instances to follow, the best posts get selected and they get posted, without being spammy. You don’t have to see everything, but only the one that both were highly-ranked on Mastodon and they were upvoted on Lemmy. At the same time, you can’t control exactly what sources are selected, but you can also interact with the community in order to drive the moderators to change the sources list, or you could just change/make a new community based on Relly. They are two different approches, that could live in symbiosis (ex. You select your own Mastodon users to follow, and the only Relly’s Mastodon posts that appear are the one that you didn’t already saw in your personal timeline [this would require collaboration with the Lemmy Server Development Team])
Some other additions:
This isn’t the same as other bots that keep on posting contents, thanks to Limits, objectives, top posts, …
Being moderated by the moderators/admin of the community/instance, the quality threshold is/should be higher.
Content is created by users who want more content. We can provide more content, and slowly stop pumping from the outside, until Lemmy is fully independent. (see Dynamic limits in my original post)
Now that I think of it, it might be a good idea to make a reverse bot, which takes the top posts from Lemmy and posts on Reddit/Mastodon/…, while providing the link to the original lemmy post, in order to drive more traffic and engagement